Portable radio telephone and methods of operation

ABSTRACT

A portable radio telephone ( 5 ) communicates with base stations ( 1, 2 ) in a digital radio telephone system, such as DECT, employing TDMA transmission. The portable radio telephone is programmed with an algorithm which is operative to select a communication channel by storing a blind slot mask ( 7 ) representative of the time slots in each frame which are excluded. The algorithm is operative to check the time slots for availability in a predetermined sequence which corresponds to the temporal sequence of the slots (or the reverse thereof) to promote orderly filling of the time slots at the base stations.

This Application is a continuation of U.S. patent application Ser. No.08/584,054, filed Jan. 11, 1996, now U.S. Pat. No. 5,862,130, issuedJan. 19, 1999, which claims priority from united Kingdom PatentApplication 9500619.3, filed Jan. 10, 1995.

BACKGROUND OF THE INVENTION

This invention relates to portable radio telephones and in particular tosuch telephones for communication with base stations in a digitalcellular radio telephone system employing transmission by a plurality ofcarrier frequencies in frames, such as TDMA frames, each consisting of apredetermined number of time slots. The invention also relates to amethod of operation of such radio telephones, frequently calledhandsets. The invention is concerned with the allocation of a channel(that is a combination of a carrier frequency and time slot) to aportable radio telephone when a connection is first required (“callset-up”) or when a change in channel is required (“handover”) during acall to maintain call quality. Handover can either be to a differentchannel at the same base station (intracell) or to a different channelat a different base station (intercell).

Most of the currently manufactured DECT base station equipment onlycontains a single transceiver, and is therefore unable to open more thana single communication channel on different frequency carriers at thesame instant. This restricts single transceiver base stations to onlyusing a single time slot at any one time, effectively excluding theother carriers on this time slot and thereby turning them “blind”. TheDECT standards have foreseen this and include the blind slot informationmessage to inform the handset of time slots it should avoid using.However, this information is not totally reliable, particularly onintercell handover when it is very difficult to obtain the blind slotinformation of any base station other than the one that the handset isconnected to. In addition to slots “blinded” by the base station, ahandset will be unable to switch communication channels to a slotimmediately adjacent to that which it is using.

This invention aims to provide a portable radio telephone, and a methodof operation, employing a dynamic channel assignment algorithm thatgives very robust performance, regardless of the availability andaccuracy of the blind slot information. In addition to this, if anidentical algorithm is deployed on every portable radio telephone in thesystem, further improvements in call blocking and call quality arelikely.

SUMMARY OF THE INVENTION

According to the invention there is provided a portable radio telephonefor communication with base stations in a digital radio telephone systememploying transmission by a plurality of carrier frequencies in frameseach consisting of a predetermined number of time slots, wherein theportable radio telephone is operative to select for the radio telephonea communication channel, that is a combination of carrier frequency andtime slot, by storing information concerning slots excluded by virtue oftransmission by the radio telephone or transmission by a communicatingbase station, and checking the slots in a predetermined sequence, whichpreferably corresponds to the temporal sequence of the slots or to thereverse of the temporal sequence of the slots, until an availablenon-excluded slot is found and a channel therein is selected.

According to another aspect the invention provides a method ofcommunicating between a portable radio telephone and base stations in adigital cellular radio telephone system employing transmission bycarrier frequencies in frames each consisting of a predetermined numberof time slots, comprising storing information concerning slots excludedby virtue of transmission by the radio telephone or transmission by acommunicating base station, and checking the slots in a predeterminedsequence, which preferably corresponds to the temporal sequence of theslots or to the reverse of the temporal sequence of the slots, until anavailable non-excluded slot is found and a channel therein is selected.

BRIEF DESCRIPTION OF THE DRAWINGS

A portion of a digital cellular radio telephone system consisting of twobase stations and a portable radio telephone (in the form of a handset)according to the invention will now be described, by way of example,with reference to the accompanying drawings, in which:

FIG. 1 is a diagram of the radio telephone system,

FIG. 2 is a flow chart representing the logic steps followed by analgorithm in the handset on call set-up,

FIG. 3 is a diagram showing an exemplary channel list stored in thehandset,

FIG. 4 is a diagram showing a channel list and an associated blind slotmask of the handset and a blind slot message from a base station, oncall set-up,

FIG. 5 is a diagram corresponding to that of FIG. 4 but for intracellhandover,

FIG. 6 shows a blind slot message on a current base station and a blindslot message on a new base station, to explain intercell handover,

FIGS. 7 to 9 are diagrams corresponding to that of FIG. 4 but forattempted (FIGS. 7 and 8) and successful (FIG. 9) intercell handover,and

FIG. 10 is a diagram illustrating how the algorithm generates an inverseblind slot mask after all time slots in the mask are marked blind.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 1, the portion of the radio system depicted comprisestwo fixed base stations 1 and 2 serving respective cells 3 and 4representing geographical areas of coverage which may be inside oroutside buildings. A user or subscriber to the system carries a portablehandset 5 which is capable of two-way radio communication with one orother of the base stations 1, 2, and with other base stations (notshown).

In the described example shown in the drawings, the radio telephonesystem conforms to the DECT (Digital European CordlessTelecommunications) standard and uses ten carrier frequencies, separatedby 1.728 MHz, within a frequency band from 1880 MHz to 1900 MHz. Thissystem divides time into TDMA frames, with each frame having a timeduration of 10 ms. Each frame is divided into 24 time slots, numberedfrom 0 to 23. Each frame is divided into two halves, the first half(slots 0 to 11) being reserved for the transmission of the base stationand the second half (slots 12 to 23) being reserved for the transmissionof the handset.

The handset 5 has processing means programmed with an algorithm which isoperative to select for the handset a communication channel, ie acombination of carrier frequency and time slot, taking into account thequietness of each channel, ie the degree of signal interference in thatchannel. To achieve this, the numbered signal strength of all channelsis monitored at regular time intervals and the measured signal strengthsare stored in the handset in a channel list, an example of which isshown in FIG. 3 in which the time axis extends horizontally and thefrequency axis extends vertically.

Referring to FIG. 3, the twelve slots 1-12 of a representative frame areshown as columns, with the ten carrier frequencies being represented byhorizontal rows. Hence, the array of FIG. 3 has 120 boxes, eachrepresenting a particular channel, ie a particular combination of timeslot and carrier frequency. Signal strength is allocated a level from 0to 12 (typically 6dB bands are used), with 0 being the quietest (ieleast interference) and 12 being the least quiet (ie most interference).Representative quietness levels for each channel are shown in FIG. 3 andthe channels that do not satisfy the quietness requirement are shownshaded. FIG. 3 also shows the base station scan. The base station scansone carrier every time frame and works through all ten carriers in tenframes.

The algorithm fulfils three theoretical principles that all improve theprobability of the portable handset assigning itself an availablechannel. These are:

1. Once a quiet channel has been attempted on a particular base stationit can be assumed that this slot is blind, and no other channel on thisslot should be attempted until all other slots have been tried first.

2. Once all the timeslots on the strongest base station have beenattempted it can be assumed that it is full to capacity, and the sameprocedure can be applied to the next strongest base station.

3. Channels that fulfill the quality criteria for use (in DECT the leastinterfered channels are assumed to fulfill this criterion) should thenbe sorted for assignment in slot order. Then if all handsets use thesame ordering for channels, slots will be blinded at approximately thesame rate on all base stations if roughly even numbers of calls exist ineach cell. This would have the effect of matching blind slot messagesfrom all the base stations in the system.

Using the quietness bands to specify signal strength, the qualitycriteria for channel assignment is as follows:

1. The quietest available channel

2. If this quietest channel cannot be attempted within the next threeframes (ie it is within three carriers of the current primary scan ofthe base station) a channel that can be attempted within three framescan be selected if it is within 2 bands of the quietest, otherwise thequietest is selected.

FIG. 4 shows at 6 the blind slot message received by the handset 5 fromthe base station 1 on call set-up. This indicates that the first threetime slots are blind. Since the handset 5 imposes no blind slots, theblind slot mask 7 on the handset corresponds to the blind slot message 6from the base station 1. Hence, the first three time slots in thechannel list 8 stored in the handset are blind. The algorithm isoperative to check availability of time slots in a sequence whichproceeds from the first time slot allocated for base stationtransmission to the last time slot allocated for base stationtransmission, before returning to the first slot. Hence, quiet channelsare checked for availability progressing from left to right in thechannel list 8 in FIG. 4, so that there is a tendency for channels to befilled in an orderly sequence which, if all handsets operate the samesystem, will promote efficient channel allocation at the base stations.This check could be in reverse order, and could commence with any timeslot.

The algorithm attempts set-up in a channel at “X” in the immediate slotafter the blind slots in the mask, if one of these channels is one ofthe quietest. Since the blind slot information on the portable iscorrect, set-up succeeds.

Next, referring to FIG. 5, it is assumed that an intracell handover isrequired because the current channel degrades in quality. In this casethe blind slot message 9 from the base station 1 indicates that thefirst three slots remain blinded, together with the fourth slot becausethis is occupied by the transmission of the handset 5. The channelcurrently occupied by the handset, together with a channel either sidethereof, is blinded by the handset, so the blind slot mask on thehandset is as shown at 10, the resulting channel list being shown at 11.

This time the channel assigned, channel X, is slightly out of slot orderon the base station 1. However, this vacant slot will soon be filled byanother handset, due to the channel assignment procedure being in slotorder. This type of algorithm (if used on every handset in the system)will tend to have the effect of assigning adjacent slots at the basestation 1. This also decreases the chance of blind slots at the handsetnot actually being blind at the base station 1.

Next, referring to FIG. 6, it is assumed that intercell handover isrequired, and that the blind slot message 12 at the new base station 2is different from the blind slot message 13 at the current base station1. This might not be the case if the proposed algorithm is used on everyportable terminal in the system.

On the handset, the current transmission channel and the two immediatelyadjacent channels are blinded so, referring to FIG. 7, the blind slotmessage from the currently connected base station 1 is shown at 14 andthe blind slot mask on the handset at 15, the resulting channel list onthe handset being as illustrated at 16. The handset tries channels inthe fourth time slot but as this is blinded at the new base station (asshown by the blind slot message 12) channel assignment fails.

Once the handset fails assignment in a channel, it then marks this slotas blind and tries channels in the next slot. As shown in FIG. 8, thehandset fails in this slot (the eighth) also, marks it as blind andtries the next slot (the ninth), as shown in FIG. 9. This time, channelassignment is successful and handover occurs.

To ensure all possible slots are attempted at least once, the followingprocedure is used. Once all the slots in the blind slot mask have beenmarked blind, an inverse of the blind slot message is used to reset theblind slot mask, and the process continues as before. This is shown inFIG. 10.

This ensures that if an available slot exists on the base station (andit is not one of those thought to be blind by the handset) that it willbe found. Once all slots have been tried at a single base station (inthis case it will take nine attempts), then an alternative base stationcan be tried. The first tried is usually the strongest, and the secondtried will be the second strongest. Once a new base station is decidedupon, the blind slot mask is reset and the procedure starts again.

The important features of the algorithm are:

1. The slot ordering of channels that meet a defined quality criteriafor channel assignment, particularly on every handset in the system.This has two beneficial effects:

Blind slots on the base stations are more likely to be the same, thusimproving the probability that portable handsets using the current basestation blind slot information can perform intercell handover to anavailable slot at other base stations.

Channels will be used at base stations that are generally immediatelyadjacent to each other, thus the likelihood of handset (that have blindslots adjacent to the one in use) missing available slots at the basestation is reduced.

2. The marking of the entire slot as blind (in addition to those alreadyblind) once assignment has failed at a single channel in this time slot.

3. When using (2), the limiting of channel assignment attempts at asingle base station to the maximum number of available slots minus thenumber of blind slots at the handset.

4. Using (3) to detect a “busy” base station and then starting thechannel assignment procedure anew with a different base station.

FIG. 2 shows the steps followed by the algorithm in call set-up.Commencing at start, box 20, the next step, box 22, is to measure signalstrengths, accept the blind slot message from a base station (ifavailable) and load into the storage means in the handset the blind slotmask. Channel selection (box 23) then proceeds from the first time slot.If an available time slot is found, channel set-up is attempted untilall nine channels are attempted, after which set-up with an alternativebase station is attempted. Each time set-up in a channel fails, the timeslot containing that particular channel is marked blind. If the blindslot mask is full (decision block 24), the original blind slot messageis inverted (box 25) and the process repeated (box 26).

What is claimed is:
 1. A portable radio telephone for communication withbase stations in a digital cellular radio telephone system employingtransmission by a plurality of carrier frequencies in frames eachconsisting of a predetermined number of time slots, wherein the portableradio telephone is operative to select for the radio telephone acommunication channel, that is a combination of a carrier frequency anda time slot, by storing information concerning time slots in a list, thelist including both non-excluded time slots and excluded time slots, theexcluded time slots being excluded by the radio telephone or by atransmission from a communicating base station, and checking one channelon each of the non-excluded time slots in the list in a predeterminedsequence until an available channel is selected.
 2. A portable radiotelephone according to claim 1, wherein the predetermined sequencecorresponds to the temporal sequence of the time slots.
 3. A portableradio telephone according to claim 1, wherein the predetermined sequencecorresponds to the reverse of the temporal sequence of time slots.
 4. Amethod of communicating between a portable radio telephone and basestations in a digital cellular radio telephone system employingtransmission by carrier frequencies in frames each consisting of apredetermined number of time slots, comprising storing informationconcerning time slots in a list, the list including both non-excludedtime slots and excluded time slots, the excluded time slots beingexcluded by the radio telephone or by a transmission from acommunicating base station, and checking one channel on each of thenon-excluded time slots in the list in a predetermined sequence until anavailable channel is selected.
 5. A method according to claim 4, whereinthe predetermined sequence corresponds to the temporal sequence of thetime slots.
 6. A method according to claim 4, wherein the predeterminedsequence corresponds to the reverse of the temporal sequence of the timeslots.
 7. A portable radio telephone for communication with a basestation in a digital cellular radio telephone system, comprising: aprocessor for selecting an available communication channel for saidportable radio telephone; a storage device coupled to said processor;and a communication channel list stored in said storage device, saidcommunication channel list being logically ordered as a plurality ofrows and a plurality of columns, wherein each row represents one of aplurality of carrier frequencies, and each column represents one of apredetermined number of time slots such that each intersection of a rowand a column represents one of a plurality of communication channels;wherein said processor makes received signal strength measurements andstores information in said communication channel list which representsan amount of signal interference in individual ones of saidcommunication channels; wherein said processor examines one channel ineach time slot in a predetermined sequence until an availablecommunication channel therein is selected for establishingcommunications with said base station.
 8. A portable radio telephoneaccording to claim 7, wherein said base station provides a time slotavailability message to said portable radio telephone which indicatescommunication channel usage, and wherein said radio telephone causessaid communication channel list to be modified to reflect thecommunication channel usage.
 9. A portable radio telephone according toclaim 8, wherein said time slot availability message is provided fromsaid base station at least during call set up and handover.